[COFF] Remove empty sections before calculating the size of section headers

The number of sections is used in assignAddresses (in
finalizeAddresses) and the space for all sections is permanent from
that point on, even if we later decide we won't write some of them.

The VirtualSize field also gets calculated in assignAddresses, so we
need to manually check whether the section is empty here instead.

Differential Revision: https://reviews.llvm.org/D54495

llvm-svn: 347704